Analysis of organic facies in Permian Lucaogou Formation in Shitoumei area of Santanghu Basin:A case from well Byy1

In order to study the types of organic facies and the controlling factors of oil shale formation in the Permi⁃ an Lucaogou Formation at the Shitoumei area of Santanghu Basin，some tests and analysis of organic carbon，rockeval，major elements，trace elements and rare earth elements were carried out based on a detailed observation of cores and a systematic sampling of mudstones. Results show that the Lucaogou Formation in the study area can be divided into four organic facies types:B，C，D and E according to its sedimentary environment，source of organic matter，preser⁃ vation condition and kerogen type. The organic facies B was formed in a semi⁃deep⁃deep lake setting，where the organ⁃ ic matters mainly from plankton with a small amount of higher plants were accumulated under a reduction condition， forming the type I and II1 kerogens. The organic facies C was formed in a shallow lake setting，where the organ⁃ ic matters characterized by a mixed source of plankton and higher plants were preserved under a reduction condition， forming the type II1 and III kerogens. The organic facies D was formed in a coastal and shallow lake setting，where the organic matters mainly sourced by higher plants were preserved under a reduction condition，forming the type III kero⁃ gens. The organic facies E was formed in a coastal and shallow lake setting，with higher plants as the main organ⁃ ic matter，an oxidation condition，and a dominating type III kerogens. Among them，oil shale was mainly developed in the organic facies B. The organic facies association of the Lucaogou Formation is D-C-B-C-E，presenting an as⁃ cending and descending vertical organic filling sequence. With the evolution of organic filling sequence，organic matter abundance is from small to large and then to small，organic matter type is III-I-III，organic matter preservation con⁃ dition is from poor to good and to poor，paleoclimate is from hot to warm and humid and then to hot，and water salinity is from fresh to brackish and to fresh. The organic facies B where the oil shale was developed is most abundant in or⁃ ganic matter and in the contribution of lake organisms，which meanwhile has a brackish water body，a warm and hu⁃ mid climate condition，and a reduced preservation condition. Under the relatively stable tectonic background of the Lu⁃ caogou Formation，the increase of water salinity caused by seawater injection and the warm and humid climate condi⁃ tions are important for the formation of oil shale. The rich nutrient elements in the brackish water body and the warm and humid climate conditions make the lake organisms flourish，and thus increase the organic matter abundance as well as the contribution of lake organisms. At the same time，the salinity stratification caused by the brackish water body and the deep-water reduction conditions provides a good preservation condition for organic matter accumulation. These factors together promote the formation of oil shale in the organic facies B.